This artificial Pavlov's dog created from a programmable liquid crystal polymer network can 'learn' to respond to light after association with heating. Photo: Zeng and Zhang et al.
This artificial Pavlov's dog created from a programmable liquid crystal polymer network can 'learn' to respond to light after association with heating. Photo: Zeng and Zhang et al.

Researchers in Finland are ‘training’ pieces of plastic to walk under the command of light. Reported in a paper in Matter, this is the first time a synthetic actuator has ‘learnt’ to do new ‘tricks’ based on its past experiences, without computer programming.

These plastics, made from thermo-responsive liquid crystal polymer networks and a coat of dye, are soft actuators that can convert energy into mechanical motion. Initially, the actuator only responds to heat, causing it to bend over, but by associating light with heat, it also learns to bend in response to light. By irradiating the actuator periodically, it is then able to ‘walk’ like an inchworm at a speed of 1mm/s, about the same pace as a snail.

"Our research is essentially asking the question if an inanimate material can somehow learn in a very simplistic sense," says senior author Arri Priimägi of Tampere University in Finland. "My colleague, Professor Olli Ikkala from Aalto University, posed the question: can materials learn, and what does it mean if materials would learn? We then joined forces in this research to make robots that would somehow learn new tricks." The research team also included postdoctoral researchers Hao Zeng at Tampere University and Hang Zhang at Aalto University in Finland.

The conditioning process, which associates light with heat, allows the dye on the surface to diffuse throughout the plastic actuator, turning it blue. This increases the overall light absorption of the actuator, which boosts the photothermal effect and raises the actuator's temperature, allowing it to ‘learn’ to bend upon irradiation.

"This study that we did was inspired by Pavlov's dog experiment," says Priimägi. In this famous experiment by Russian physiologist Ivan Pavlov, a dog salivated in response to seeing food. Pavlov then rang a bell before giving the dog food. After a few repetitions, the dog associated food with the bell and started salivating upon hearing the bell.

"If you think about our system, heat corresponds to the food, and the light would correspond to the bell in Pavlov's experiment," explains Priimägi.

"Many will say that we are pushing this analogy too far. In some sense, those people are right, because compared to biological systems, the material we studied is very simple and limited. But under right circumstances, the analogy holds."

The next step for the team is to increase the level of complexity and controllability of the system, in order to find the limits of the analogies that can be drawn to biological systems. "We aim at asking questions which maybe allow us to look at inanimate materials from a new light."

Besides walking, the system can also ‘recognize’ and respond to different wavelengths of light that correspond to the coating of its dye. This characteristic makes the material a tunable soft micro-robot that can be remotely controlled, which could be ideal for biomedical applications.

"I think there's a lot of cool aspects there. These remotely controlled liquid crystal networks behave like small artificial muscles," says Priimägi. "I hope and believe there are many ways that they can benefit the biomedical field, among other fields such as photonics, in the future."

This story is adapted from material from Cell Press, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.